1. Field of the Invention
The present invention relates to a method of controlling an electric braking operation of a linear motor which is controlled by a variable voltage variable frequency (VVVF) inverter and, more particularly, to a control method which, through simple control, enables electric braking operation to be effectively utilized to bring the linear motor substantially to a stop.
2. Description of the Related Art
FIG. 1 is a characteristic chart of electric braking operation of a rotary type induction motor, which is described in, for example, "RAILWAY ELECTRIC ROLLING STOCKS" (Denkisha no Kagaku), vol. 39, No. 6, 1988, page 30. In the figure, symbol BE denotes an electric braking force, and symbol I denotes a motor current. During the stopping period of the induction motor, after the speed V of a vehicle decreases to V.sub.1 (for example, approximately 10 km/h), the electric braking force BE is decreased by decreasing the motor current I. When the speed V reaches a speed V.sub.3 immediately before the stoppage of the induction motor (for example, 3-5 km/h), then the electric braking force BE is set to zero. In the speed region less than the speed V.sub.1, the stopping of the vehicle is controlled by increasing an air braking force so as to obtain a constant overall braking force including the electric braking force BE and the air braking force.
FIG. 2 is a characteristic chart showing a conventional method of controlling electric a linear motor, which is disclosed in Japanese Patent Laid-open No. 59136087. As can be seen from the figure, a slip frequency fs is kept at a constant value fs.sub.0 in a variable voltage variable frequency (VVVF) region in which the speed V of the linear motor ranges from V.sub.4 to V.sub.2 and, in a constant voltage variable frequency (CVVF) region greater than V.sub.2, the slip frequency fs increases in accordance with an increase in the speed V.
In general, during the operation of the electric braking operation, the following relationship is established EQU fm=fi+fs (1)
where fm is the speed frequency obtained by converting the speed V into a frequency and fi is the inverter output frequency. The electric braking force BE depends upon the inverter output frequency fi and, when the inverter output frequency fi reaches 0 Hz, the electric braking force BE loses its effect. Accordingly, where, as in the case of the conventional example of FIG. 2, the electric braking force is applied while controlling the slip frequency fs to be the constant value fs.sub.0 in the VVVF region, when the speed frequency fm becomes equal to the slip frequency fs.sub.0, the inverter output frequency fi becomes 0 Hz in accordance with the equation (1) and the electric braking force BE loses its effect. However, at this point in time, since the speed frequency fm is not 0 Hz, it follows that the linear motor has a speed V.sub.4 which is represented by EQU V4=fm.multidot.K=fs.sub.0 .multidot.K
where K is a constant which is used to convert the frequency into a speed.
In the region of less than the speed V.sub.4, the stopping of the vehicle is controlled by means of a brake, such as an air brake, which utilizes friction. In general, however, since the slip frequency fs of the linear motor is large relative to the inverter output frequency fi, the speed V.sub.4 at which the electric braking force BE loses its effect is, for example, 10-15 km/h which is high compared to the speed V.sub.3 (3-5 km/h) of the general rotary type induction motor described above.
In practice, the electric braking force BE is intentionally decreased and the air brake is started to act at a speed higher than the speed V.sub.4 (for example, 20-25 km/h) so as to gradually shift the stopping control by means of the electric braking force BE to one by means of the air braking force. Accordingly, use of the air brake utilizing friction must be initiated at fairly high speeds, thus resulting in the problem that skidding easily occurs. If skidding occurs and the contact surfaces of wheels become flat, noise between wheels and rails may occur and the vehicle becomes uncomfortable to ride in. In addition, the labor required for maintenance, such as grinding of the contact surfaces of the wheels, may increase.
In order to prevent the above-described skidding, a method is proposed which comprises the steps of activating a reverse-phase braking operation to slow down the linear motor in the speed region not greater than the speed V.sub.4 at which the electric braking force BE loses its effect, and then using the air brake after the linear motor has slowed down. In the reverse-phase braking operation, when the inverter output frequency fi reaches 0 Hz, the phase sequence of the inverter output is reversed to apply a braking force which acts in the direction opposite to the preceding direction of vehicle travel. Accordingly, if the reverse-phase braking operation is to be used, in order to prevent the vehicle from travelling backwardly, it is necessary to detect the fact that the speed V of the linear motor has reached 0 km/h and then stop the linear motor. As described above, in case that the reverse-phase braking operation is to be used, it is required to use a complicated control for realizing the change of the phase sequence of the inverter output, the detection of zero speed, or the like.